European patent publication 239,190 discloses a measuring cell for hydride gases. Typical hydride gases are diborane, silane, phosphine and arsine. Hydride gases are applied, for example, in the manufacture of semiconductors.
In this known measuring cell, the reaction of the substance to be detected, namely the hydride gas, takes place on the surface of the measuring electrode. For this purpose, the substance to be detected must diffuse from the gaseous phase outside of the measuring cell through the electrolyte to the surface of the electrode before the reaction which leads to a measuring signal can take place.
A low concentration gradient of the substance to be detected is formed from the gaseous phase to the electrode surface because of the long diffusion path. This leads to a low sensitivity because the substance to be detected dwells too long in the electrolyte layer before it is transported away by means of the appropriate reactions.
The heterogeneous electron transfer on the electrode surface leads generally to a reduced selectivity for the hydride gases to be investigated. Furthermore, the action of extraneous material in the substance to be detected on the surface of the measuring electrode can lead to a premature poisoning. This causes the following: the diffusion time becomes longer, the sensitivity is reduced, and the number of molecules of the substance to be detected which are converted at the electrode and which diffuse through the membrane per unit of time becomes less. All these disadvantages effect an unsatisfactory long-term stability.